The present invention relates to a planar trimming resistor having a substrate and a resistive layer deposited thereon; a resistance bridge circuit and a sensor which make use of a trimming resistor of this kind; and a method for manufacturing the trimming resistor, the resistance bridge circuit, or the sensor.
Trimming resistors are used in circuits which are initially produced with a certain tolerance, and whose response is subsequently adjusted accurately by adjusting the resistance value of the aforementioned trimming resistors. Generally known are trimming resistors having a planar resistive layer whose resistance value can be adapted, subsequent to installing the trimming resistor in a circuit, by locally removing material of the resistive layer. In this context, the problem arises that a change in the resistance value by removing material of the resistive layer also influences the capacitance thereof, especially when the resistive layer extends in the immediate vicinity of a further conductor, for example, because the resistive layer is deposited on a metallic substrate in a manner that it is separated only by a thin insulating layer. Such a change in capacitance can destroy the value of the trimming if the response of the overall circuit is not only influenced by resistance value but also by the capacitance of the trimming resistor. This problem always arises when the trimming resistor is part of a resonant circuit.
The present invention provides a planar trimming resistor whose capacitance is not influenced by the trimming process.
This advantage is achieved in that a contact layer having a better conductivity than the resistive layer is arranged on the resistive layer so that it is in conducting contact with the latter at least at discrete or isolated locations, and in that the contact layer is accessible to an abrasive treatment at least locally. When the contact layer in such a trimming resistor is locally removed, then the result is that a current which would otherwise have flown through the contact layer, is deflected into the resistive layer at least partially so that the resistance value is increased in a controlled manner by removing the contact layer. However, since the resistive layer remains under the removed contact layer, the electrically conductive surface of the trimming resistor located opposite the substrate does not change and its capacitance is not changed by the removal process.
In principle, the contact layer can extend on the substrate beyond the boundaries of the resistive layer. However, those regions of the contact layer which are not lying above the resistive layer must not be removed since the surface of the trimming resistor would otherwise be reduced, thus unwantedly influencing the capacitance. To rule out problems of this kind, it is expedient for the contact layer to be dimensioned during the manufacture of the trimming resistor in such a manner that it does not extend beyond the edges of the resistive layer.
The contact layer and the resistive layer have preferably a strip-shaped design. Connection terminals are provided at opposed ends of the contact layer strip. The strips can be arranged on the substrate in a space-saving manner in a zigzag or meander form. The trimming resistor has preferably a passivation layer which essentially covers the resistive layer and has windows only locally which make the conductive contact of the contact layer with the resistive layer possible.
Between two windows of the passivation layer, in each case provision is preferably made for a narrow spot of the contact layer. At such a narrow spot, the contact layer can be cut through particularly easily; the cutting through of the contact layer resulting in that an electric current which would otherwise have flown through the contact layer must follow the path via the resistive layer between the two windows.
The passivation layer not only serves for protecting the resistive layer from environmental influences; its window pattern has the advantage that is sufficient for the contact layer to be cut through over a small length between two windows for extending the current path in the resistive layer; otherwise compared to this length a larger distance between two windows would be needed.
A preferred application of the trimming resistor is a resistance bridge circuit. In such a resistance bridge circuit, the resistor elements and the at least one trimming resistor are expediently formed above the same substrate. In this manner, the resistor elements and the trimming resistor can partly be manufactured using the same process steps.
A further preferred application of the trimming resistor is a sensor having a deformable substrate and at least one resistor element whose resistance value is variable by a deformation of the substrate, and to which a trimming resistor according to the present invention is allocated. This resistor element, in turn, can be part of a resistance bridge circuit.
The sensor can advantageously be a pressure sensor. The substrate can be part of a pressure capsule of such a pressure sensor.
Furthermore, the present invention relates to a method for manufacturing a trimming resistor, where a resistive layer is deposited and patterned on an insulating substrate, and where a contact layer having a better conductivity than the resistive layer is formed above the resistive layer, the contact layer being in contact with the resistive layer at least locally.
A passivation layer having windows for making the local contact is preferably deposited on the resistive layer prior to forming the contact layer. Suitable for patterning the different layers are photolithographic techniques, or, particularly advantageous in the case of sensors, are laser patterning techniques. In particular, the contact layer can be produced by sputter deposition of the layer material as well. It is also conceivable for the material of the contact layer to be directly deposited through a mask so that the contact layer is formed on the resistive layer (separated by the passivation layer) immediately in the desired shape.
The resistor is trimmed preferably by cutting through the contact layer between two windows, preferably using laser ablation. This laser ablation can be carried out using the same apparatuses which have possibly been used before for the laser patterning of the different layers.